The present invention relates to a laser machining apparatus capable of producing a high quality laser beam.
FIG. 38 illustrates a conventional laser machining apparatus for use in improving the surface quality of a metal workpiece, thickening a metal workpiece or drying wet paper at high speed. In FIG. 38, a reference numeral 1 depicts a full reflection mirror, 3 a laser medium, 4 an output mirror, 7 a laser beam generated in a laser resonator 6 constituted with the full reflection mirror 1 and the output mirror 4, 8 an output laser beam derived from the laser resonator 6, 9 a bend mirror, 8a a laser beam bent by the bend mirror 9 and 10 a workpiece. Further, a reference numeral 5 depicts a non-reflection coating formed on an outer surface of the output mirror 4 and 5a a partial reflection coating formed on an inner surface of the output mirror 4 to make the output mirror 4 partially reflective.
In operation, the laser beam 7 reciprocating between the full reflection mirror 1 and the output mirror 4 is amplified by the laser medium 3 and a portion thereof is derived from the output mirror 4 as the output laser beam 8 having power distribution such as shown in FIG. 39. The laser beam 8 is guided by the bend mirror 9 to the workpiece 10 to improve the surface quality thereof or thickening it when the workpiece 10 is a metal workpiece or drying it at high speed when the workpiece is wet paper.
It is usual in such laser machining apparatus to increase the output power by making the laser medium large so that a higher speed machining is realized. In such a case, however, in order to derive the output laser beam efficiently from the laser medium, it is necessary to make a cross sectional area of the laser beam 7 in the resonator 6 large. The intensity distribution : transverse mode thereof becomes the so-called higher mode described in, for example, "Basic Optoelectronics", Maruzen, 1974.
FIG. 40 illustrates an example of a laser machining apparatus for cutting and/or welding workpieces which is disclosed in M. Hamazaki, "Practical Laser Machinings", Tech Publishing Co., Feb. 20, 1986. In FIG. 40, a laser beam 8 derived from a laser resonator 6 is guided by a bend mirror 9 to a condenser lens 11 as a laser beam 8a to obtain a high power concentration and machines, together with laser gas 23 guided from a gas inlet port 22, a workpiece 10
In order to realize a high performance laser machining, a high quality, high power laser beam is required. The high quality requirement is achieved in the apparatus in FIG. 40 by limiting an outer diameter of the laser beam by means of an aperture 21. When the diameter of the aperture is small enough, a Gaussian beam is produced as shown in FIG. 41a. Since such beam can be condensed by the condenser lens 11, a high power concentration laser beam is obtained as shown in FIG. 41b. On the other hand, when the diameter of the aperture 21 is large, a condensation of the laser beam is degraded, resulting in a ring shaped laser output having high multi modes as shown in FIG. 41c.
FIG. 42 is a graph showing a cutting performance of the apparatus in FIG. 40 when an iron plate is cut thereby, in which a curve A corresponds to a case when the cutting is performed with multimode laser beam of 1000 W and B corresponds to a cutting with Gaussian-mode laser beam of 500 W. As is clear from FIG. 42, the cutting performance B is superior to A although the laser power for A is larger than that for B. That is, the mode quality of laser beam is very important in laser machining.
FIG. 43 illustrates another example also shown in "Practial Laser Machinings" mentioned above. In FIG. 43, a laser beam 7 reciprocating between an output mirror 4 and a full reflection mirror 1 of a resonator is amplified by a laser medium 3 and, when an intensity of the beam exceeds a predetermined level, a portion thereof is derived as a laser beam 8 which is condensed by a condensing or machining lens 11 and a resultant high power beam is directed to a workpiece 10 to machine it.
In order to machine the workpiece 10 efficiently, an output power of the laser 8 must be derived stably from the output mirror 4. In order to realize the latter, it is necessary to lower the beam intensity by making a cross sectional area of the beam 7 larger. However, the laser beam 8 derived from the laser beam 7 and having large cross section is of multimode having an intensity distribution having substantially peaks at both ends thereof such as shown in FIG. 44a. When such beam is condensed by the machining lens 11, an intensity distribution of a resultant laser beam 8b has very sharp peaks at opposite ends thereof as shown in FIG. 44b.
FIGS. 45 and 46 show examples of a conventional composite laser machining apparatus such as disclosed in Japanese Patent Application Laid open No. 4282/1985 when applied to an welding and a hardening, respectively.
In FIG. 45, an expanding full reflection mirror 12 and a full reflective collimating mirror 1 constitute a unstable resonator 6 and a laser beam 15 reciprocating therebetween is amplified by a laser medium 3. An outer peripheral portion of the laser beam 15 is derived from a opened mirror 13 of the laser resonator 6 every reciprocation and after passed through a window 14 it is obtained as a ring shaped laser beam 15a which is condensed by a lens 11 and then directed to a workpiece 10.
In FIG. 46, a full reflection collimating mirror 1, a bend mirror 9 and a partial reflective mirror 14a constitute a stable resonator 6 and a portion of a laser beam 15 reciprocating between the collimating mirror 1 and the partial reflection mirror 14a is derived from the partial reflection mirror 14a and after condensed by a lens 11 directed to a workpiece 10.
FIG. 47a shows an intensity distribution of a laser beam 15a produced by the device shown in FIG. 45 and FIG. 47b shows an example of beam pattern of the laser beam 15a after condensed. That is, a ring shaped laser beam shown in FIG. 47a is condensed as a filled in beam having side peaks such as shown in FIG. 47b, due to an effect of diffraction. Since the pattern of the condensed beam has a center peak which is substantially high, it is effectively used in welding a workpiece 10, etc. However, due to such localized power concentration, it can not be used for surface treatment of workpiece which requires a uniform distribution of power. Further, since such ring shaped laser beam changes its pattern considerably around a focal plane, it is hardly to use such beam in cutting the workpiece which requires a large focal length.
FIGS. 48a, 48b and 48c show examples of variation of beam pattern shown in FIG. 47b when a ratio of outer diameter of the ring shaped laser beam which is 20 mm to inner diameter is 2 and a condenser lens has a focal distance 400 mm. FIG. 48a shows the pattern at a position on front side of the focal plane remote therefrom by 20 mm, FIG. 48b shows that at the focal plane and FIG. 48c shows at a position on rear side of the focal plane remote therefrom by 20 mm. As is clear from FIGS. 48a to 48c, the height of center peak is reduced on both sides of the focal plane and the side peaks are broadened.
In general, in a welding which requires high power concentration, it is considered that only laser power included in the center peak portion is used. In such case, calculation of percentage of power contained in the center peak to total power shows about 30% in the case of FIG. 48a, about 50% in the case of FIG. 48b and about 30% in the case of FIG. 48c. Therefore, it can be said that a distance in which a certain constant efficiency of laser machining, i.e., the focal depth, is small due to such change of intensity distribution pattern around the focal plane. For this reason, such ring shaped laser beam is not used for laser cutting.
For a surface treatment of workpiece 10, the apparatus is usually constituted with a stable resonator such as shown in FIG. 46 in which the mirror 13 having the hole and used in FIG. 45 is replaced by a bend mirror 9 and the window 14 is replaced by a partial reflection mirror 14a. In such construction, a laser beam from the resonator 6 becomes a filled-in pattern having peaks at both ends thereof as shown in FIG. 49a and, after condensed by a lens 11, a similar pattern is provided as shown in FIG. 49b. Although this pattern has no center peak, the peaks at both sides thereof causes a surface treatment to be non-uniform.
Thus, in the conventional laser machining apparatus, that shown in FIG. 38 can not be used for a uniform machining due to the fact that the higher mode laser beam has the intensity pattern such as shown in FIG. 39 which provides very high power portions at both ends thereof and that shown in FIG. 40 is not suitable to produce an enough power from the laser medium 3 efficiently due to the fact that the small aperture 21 is used to make the laser beam high quality. In the latter case, when the power is increased, the lens 11 may be deformed or damaged in some case since the intensity of beam fallen in the lens becomes very high Further, in the apparatus shown in FIG. 43, it is necessary to use a laser beam 8 whose intensity distribution pattern has very high peaks at both ends thereof. Therefore, the machining itself becomes non-uniform. Further, since the output mirror 4 is heated non-uniformly, the laser output becomes unstable with increase of the output power, resulting in damage of the mirror.
Since, in the composite laser machining apparatus such as shown in FIG. 45 or 46, the resonator 6 must be changed according to the kind of machining, which takes a long time and requires complicated preparation stage. In addition, the area to be machined is limited.